[turbofan] Add initial support for global specialization.

src/compiler/js-context-specialization.cc

Index: src/compiler/js-context-specialization.cc
diff --git a/src/compiler/js-context-specialization.cc b/src/compiler/js-context-specialization.cc
index 0ad25e179d7ecea6f5d1c7718a12e5537fdeb751..a4f3ff49864fd9815c6e47bf9265c343b09019fa 100644
--- a/src/compiler/js-context-specialization.cc
+++ b/src/compiler/js-context-specialization.cc
@@ -95,6 +95,9 @@ Reduction JSContextSpecialization::ReduceJSLoadContext(Node* node) {
   // Success. The context load can be replaced with the constant.
   // TODO(titzer): record the specialization for sharing code across multiple
   // contexts that have the same value in the corresponding context slot.
+  if (value->IsConsString()) {
+    value = String::Flatten(Handle<String>::cast(value), TENURED);
+  }
   Node* constant = jsgraph_->Constant(value);
   ReplaceWithValue(node, constant);
   return Replace(constant);